1. Field of the Invention
The present invention relates to a ceramic heater for use in a glow plug for preheating a diesel engine or a like device, and to a glow plug using the same.
2. Description of the Related Art
A conventionally known ceramic heater for the above-mentioned applications is configured such that a resistance-heating member formed of an electrically conductive ceramic is embedded in an insulating ceramic substrate. In such a ceramic heater, electricity is supplied to the resistance-heating member via metallic leads formed of tungsten or a like metal. However, use of the metallic leads involves a corresponding increase in the number of components, possibly resulting in an increase in the number of manufacturing steps and thus an increase in cost. In order to cope with the problem, Japanese Patent No. 3044632 discloses an all-ceramic-type heater structure, in which a first resistor portion serves as a major resistance-heating portion, and a second resistor portion formed of an electrically conductive ceramic having an electrical resistivity lower than that used to form the first resistor portion serves as an electricity conduction path to the first resistor portion, thereby eliminating the need for metallic leads.
Integration of resistor portions of different electrical resistivities facilitates implementation of a ceramic heater having a so-called self-saturation-type heat generation characteristic; i.e., a ceramic heater which functions in the following manner: at an initial stage of electricity supply, large current is caused to flow to the first resistor portion via the second resistor portion to thereby increase temperature promptly; and when the temperature rises near to a target temperature, current is controlled by means of an increase in electric resistance of the second resistor portion. Japanese Patent Application Laid-Open (kokai) No. 2000-130754 also discloses this effect as well as a ceramic heater structure in which electricity is supplied, via metallic leads, to a ceramic resistor configured such that two resistor portions of different electrical resistivities are joined together.
3. Problems to be Solved by the Invention
In ceramic heaters having the structure disclosed in the above-described patent publication, a joint interface between ceramic resistors formed of different materials is inevitably formed. Usually, electrically conductive ceramics of different electrical resistivities differ considerably from each other in coefficient of linear expansion. Accordingly, in an application involving frequent repetition of temperature rise and cooling as in the case of a glow plug, thermal stress induced by the above-mentioned difference in coefficient of linear expansion tends to concentrate at the joint interface between resistor portions of different kinds. Particularly, in the case in which a sufficiently large joint area cannot be secured, a problem arises in that strength becomes insufficient, and sufficient durability cannot be secured.
It is therefore an object of the present invention to provide a ceramic heater which exhibits excellent durability even though its ceramic resistor assumes the form of a joined body consisting of resistor portions of different kinds, as well as a glow plug using such a ceramic heater.
The above-described problems, of the prior art have been solved by providing a ceramic heater of the present invention comprises a rodlike heater body which is configured such that a ceramic resistor formed of an electrically conductive ceramic is embedded in a ceramic substrate formed of an insulating ceramic, and is configured such that a ceramic resistor formed of an electrically conductive ceramic is embedded in a ceramic substrate formed of an insulating ceramic. The ceramic heater is characterized in that the ceramic resistor comprises a front end part disposed at a front end portion of the heater body and is formed of a first electrically conductive ceramic, and two large-diameter rodlike portions joined to two end parts of the front end part as viewed along a direction of electricity supply and forming passages for supplying electricity to the front end part. Each of the large-diameter rodlike portions extends rearward along a direction of an axis of the heater body and has an electricity-supply sectional area greater than that of the front end part. Each of the large-diameter rodlike portions has a connection end part connected to the front end part. The connection end part is formed of the first electrically conductive ceramic and constitutes a first resistor portion in cooperation with the front end part. The remaining portion of each of the large-diameter rodlike portions is formed of a second electrically conductive ceramic having electrical resistivity lower than that of the first electrically conductive ceramic and constitutes a second resistor portion. A joint interface between the first resistor portion and the second resistor portion is located within the corresponding large-diameter rodlike portions.
The glow plug of the present invention comprises the above-described ceramic heater of the invention; a metallic sleeve disposed so as to circumferentially surround the heater body of the ceramic heater and such that a front end portion of the heater body projects therefrom along the direction of the axis; and a metallic shell joined to a rear end portion of the metallic sleeve as viewed along the direction of the axis and having a mounting portion formed on an outer circumferential surface thereof, the mounting portion being adapted to mount the glow plug onto an internal combustion engine.
In the above-described ceramic heater, since the front end part of the ceramic resistor has a reduced diameter, current intensively flows to the front end part, which assumes the highest temperature during operation. Therefore, a compact ceramic heater which can generate a large amount of heat can be obtained. In the present invention, the ceramic resistor assumes the form of a joined body consisting of first and second resistor portions. As described above, the joint interfaces are those of ceramic resistors formed of different materials. Accordingly, in an application involving frequent repetition of temperature rise and cooling as in the case of a glow plug, thermal stress induced by the difference in coefficient of linear expansion between the two ceramics tends to concentrate at the joint interface. However, in the present invention, by utilizing the unique configuration of a resistor in which the diameter is reduced locally at its front end part, the above-described joint interface is formed at the large-diameter rodlike portion in order to effectively increase the joint area. As a result, the margin for strength against thermal stress concentration can be increased, whereby a ceramic heater having excellent durability can be realized. Moreover, positioning of the joint interface at the large-diameter rodlike portion means that the joint interface is not formed at the small-diameter front end part. Therefore, the distance between the joint interface and the front end position of the ceramic resistor, where temperature rises to the highest level by heat generation, can be increased accordingly, thereby restraining the joint interface from being subjected to an excessively great temperature gradient and heating-cooling cycles of great temperature hysteresis.
In the claims appended hereto, reference numerals identifying components are cited from the accompanying drawings for a fuller understanding of the nature of the present invention, but should not be construed as limiting the concept or scope of the components in the claims.